Front injector bank not firing (1976 280Z)

[NicholasKoenig]

Recently bought a Z and am sorting out the issues. 

It runs albeit on the 3 rear cyclinders. New fuel pump, new NGK wires and plugs, distributor looks like it was replaced not long ago. Coil looks original (new MSD blaster in the mail). 

I know it's getting fuel, spark and air. I know all 6 injectors are functional. Just not all at the same time. 

When I crank it, it starts almost instantly, when I place my screw driver to each injector, only the rear bank makes the clicking. Removing any of the front 3 injector pig tails doesn't affect idle. 

When i place the rear 3 injector connections on the front 3, once again it starts and idles the same. 

Is this a fault in the first bank wiring harness? A dropping resistor issue, faulty ECU? 

Currently I only have the FSM available on my phone and I'm diving through it but I feel overwhelmed. 

I checked the pins with a multimeter and each injector is getting current. Did I miss something simple? 

 

Thank you for reading. Hope to hear from you guys soon. 

 

PS, I bought all new injector connections, but that doesn't seem to be the issue despite them being worn. Haven't installed yet. 

Edited November 16, 2016 by NicholasKoenig

[Zed Head]

21 minutes ago, NicholasKoenig said:

 

 

When i place the rear 3 injector connections on the front 3, once again it starts and idles the same. 

Is this a fault in the first bank wiring harness? A dropping resistor issue, faulty ECU? 

You were almost to a good conclusion.  When you put the 3 rear connections on the 3 fronts, did the 3 front cylinders start working instead of the back 3?  Or did the same cylinders work, just with different connections?  If the cylinders that work follow the connections, then it's a connection problem.  If they don't, it's a spark or mechanical problem.

These engines will start and run with the firing order of the plug wires backward.  They sound like they're running on 3 cylinders when you do that (by accident, of course).

Finally, history of the engine helps.  Did it ever run right, as far as you know?

Edit - sorry for the ad.  It came with the edit box.

 

Edited November 16, 2016 by Zed Head

[NicholasKoenig]

46 minutes ago, Zed Head said:

You were almost to a good conclusion.  When you put the 3 rear connections on the 3 fronts, did the 3 front cylinders start working instead of the back 3?  Or did the same cylinders work, just with different connections?  If the cylinders that work follow the connections, then it's a connection problem.  If they don't, it's a spark or mechanical problem.

These engines will start and run with the firing order of the plug wires backward.  They sound like they're running on 3 cylinders when you do that (by accident, of course).

Finally, history of the engine helps.  Did it ever run right, as far as you know?

Edit - sorry for the ad.  It came with the edit box.

 

The previous owner said it "ran 2-3 months ago" from the time I bought it. To my knowledge his son was wrenching on it and his work seemed lack luster but it appears as though he didn't alter any of the presumed electrical issues im experiencing rn. 

I reversed the bank connections. Placing the rear bank (the functioning one) onto the front and the front onto the rear. 

The front injectors began clicking and rear were silent. To me this rules it electrical but to what extent? Is it a wiring issue or something like the dropping resistor or ecu? 

Another thing I noticed, the part of the EFI harness that is in the engine bay looks untouched. But under the dash you can see electrical tape around the harness (didn't look that fresh but def not stock). That led me to the fuel / afm relays. The stock wires plugged into a Bosh black box that looked modern ( I doubt Japan used Bosch in the 70s?). Is this a clue? 

It seems like the harness is the issue but I don't want to mess with anything I don't need to. I'm learning along the way and any advice helps. 

Thank you for the quick reply. 

[Zed Head]

You can check for power at the injectors with the key On, with a meter or test light.  I think that power is supplied in a set of two and four.  But it's grounded through the ECU in sets of three.  So you might have an issue with the ECU.

If you have a meter, going through the tests in the EFI Guide is the way yo do things.  It's what's happening at the eCU that really matters and most of the tests start there.

The 1980 EFI guide (in the Downloads section) covers all years up to 1981.  Use that one.

 

[NicholasKoenig]

12 minutes ago, Zed Head said:

You can check for power at the injectors with the key On, with a meter or test light.  I think that power is supplied in a set of two and four.  But it's grounded through the ECU in sets of three.  So you might have an issue with the ECU.

If you have a meter, going through the tests in the EFI Guide is the way yo do things.  It's what's happening at the eCU that really matters and most of the tests start there.

The 1980 EFI guide (in the Downloads section) covers all years up to 1981.  Use that one.

 

Ok I'll go through the ECU pins again tomorrow after work. I only get about an hours worth of tinkering each day max so process is slow going. 

Yes, looking through the FSM wiring diagrams I have saved, it looks like the the first four injectors connected to one dropping resistor and the remaining 2 are on the other resistor. From this I think it's safe to assume my dropping resistor is functional. Otherwise, only four or two cyclinders would fire by My logic and understanding of the diagram. 

And since it gets fuel, spark, and air with working injectors, the only thing left is faulty harness (the first 3 injector wires) or ecu? 

Going to look over your attachment now. Ill let you know how I do with the multimeter tomorrow! 

 

[NicholasKoenig]

Turns out I have a similar attachment to yours. Instead mine is soley for the 1976 EFI trouble shooting instead of the broader range of yours. 

But looking through it helped me find the name of the black Bosch part i was questioning. It's the fuel injection relay. (Photo attached)

Perhaps this Bosch replacement is faulty. Which leads me back to the harness that was retapped under the dash. Maybe that's hiding some shoddy soldering work or other gremlins. 

 

[Zed Head]

That looks like the replacement relay for the original JECS relay.  Which is actually based on the Bosch system.  It's actually two relays in one, EFI and fuel pump.

Despite all of the pins it's a fairly simple relay.  If you get the readings that you should at the ECU connector, don't worry about it.  Many people "assume" and replace, it's hard to resist. 

If you get tricky, you can ground the injectors individually at the ECU connector, just like it would, to test the complete circuit of wires, power and injectors.  I made some male pins to use in the connector from a flattened piece of solid core copper wire.  Makes it easy to get an alligator clip connected to supply power and ground.

Forgot to say - there's a procedure for testing the EFI relay, if you get odd numbers.  In the FSM, Engine Fuel chapter, and probably in that EFI book too.

Edited November 16, 2016 by Zed Head

[Captain Obvious]

I'm thinking ECU. The ECU has two big output transistors that fire the injectors in banks of three, and if one of those transistors burns open, you will lose those three injectors.

There aren't a lot of other things that could cause you to drop three injectors like that at the same time. As you already noticed, the dropping resistor does four and two so that's probably not it. And while it is  possible that the wiring to the front three of the injectors is screwed up inside the harness, I consider it unlikely. Why? Because each injector gets it's own independent wire from the ECU to the injector. So sure... It's possible that a PO cut three of those wires, but I consider it more likely that the ECU is the root cause.

I'm in the Philly area if you get stuck. I'm no FI expert, but I know some stuff. PM me if you need a hand.

[NicholasKoenig]

Hey guys, thanks for the replies. I just got done using my dmm on the ECU pins. Injectors plugged in, key on; all six injectors read 11.92V!!! 

Does this rule out my harness and point to the ECU? 

 

[Captain Obvious]

If you got 11.9 on all six injector pins at the ECU with the ECU disconnected, then yes... It points away from a harness issue and towards the ECU.

[NicholasKoenig]

41 minutes ago, Captain Obvious said:

If you got 11.9 on all six injector pins at the ECU with the ECU disconnected, then yes... It points away from a harness issue and towards the ECU.

Gotcha. This is a relief because I am likely close to the solution instead of scratching my head at this. 

Id rather not buy an ECU from eBay, they are cheap but probably risky. I saw this one available, once the core charge is accounted for it seems like a good deal ($124).

Any vendors you guys recommend for an ECU? 

[rcb280z]

I would put an add in the classified section of this forum, never know what you might find. I had one that got another member out of a jamb. You might be as lucky.

I like MSA but sometimes their prices are.......